There is known a technique of image stereopsis, which enables stereoscopic visual perception of an image by displaying on a monitor a left-eye image and a right-eye image which have an angular difference. The image stereopsis can make it possible for a user to easily recognize a front-and-rear relation of objects and information of irregularities on surfaces of objects, which are difficult to recognize with ordinary two-dimensional displays. In recent years, the technique of image stereopsis has begun to be applied to the medical field. With the technique of image stereopsis being applied to medical treatment, for example, even in a state in which the position of a diseased part and the position of a blood vessel overlap, a user can grasp the positional relationship thereof. By stereoscopically viewing a target part, the user can perform, for example, a safe, high-precision operation. In order to enable the user to view stereoscopic video relating to a target part, it is necessary that a left-eye image and a right-eye image, which have an optimal angular difference, be displayed on the monitor.
However, the optimal angular difference of each individual user varies depending on the inter-pupil distance of the user, the distance between the user and the monitor, and the sensitivity of the user. For example, in operations and medical treatments, since a plurality of persons are involved, there is a case in which the user who views the monitor changes from time to time. In addition, there is a case in which the distance from the user to the monitor varies due to the movement of the monitor and the movement of the user. Thus, there is such a problem that the stereoscopic video, which is displayed on the monitor, can be viewed by only a limited person or within a limited range.